serdev: add controller runtime PM support

Add support for controller runtime power management to serdev core. This
is needed to allow slave drivers to manage the runtime PM state of the
underlying serial controller when its driver, in turn, implements more
aggressive runtime power management (e.g. using autosuspend).

For some applications, for example, where loss off initial data after a
remote-wakeup event is acceptable or where rx is not used at all,
aggressive serial controller runtime PM may be used without further
involvement of the slave driver. But when this is not the case, the
slave driver must be able to indicate when incoming data is expected in
order to avoid data loss.

To facilitate the common case, where the serial controller power state
is active whenever the port is open (which is the case with just about
every serial driver), and where data loss is not acceptable and cannot
even be prevented by explicit controller runtime power management, an
RPM reference is taken in serdev open and put again at close. This
reference can later be balanced by any serdev driver which wants and/or
can handle aggressive controller runtime PM.

Note that the .ignore_children flag is set for the serdev controller to
allow the underlying hardware to idle when no I/O is expected, regardless
of the slave device RPM state.

Acked-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Johan Hovold <johan@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>